1. Field of the Invention
The present invention relates to a process for producing aluminum nitride.
2. Description of the Related Art
Aluminum nitride (i.e., AlN) is good in terms of characteristics, such as heat resistance, thermal conductivity and electric conductivity, its applicable uses have been expanding. As for the applicable uses of aluminum nitride, it is possible to name containers for high-temperature applications, IC substrates, and so forth.
Industrial aluminum nitride is artificial ceramic which does not exist in nature. It is produced mainly by using two methods, the carbon reduction nitriding method of alumina and the direct nitriding method of aluminum.
(a) Carbon Reduction Nitriding Method of Alumina(i.e., Al2O3+3C+N2+2AlN+3CO)
The carbon reduction nitriding method of alumina is a production process in which alumina having a high purity is reduced at a high temperature (e.g., from 1,700 to 1,900° C.) by graphite and a nitrogen gas and simultaneously aluminum formed by reducing is nitrided by nitrogen, thereby producing aluminum nitride.
However, it takes a long time for the carbon reduction nitriding method to complete the reaction. Accordingly, the production cost goes up so that a problem arises in that the resulting aluminum nitride is more expensive than the other ceramics, for example, silicon carbide (i.e., SiC), alumina, and so on.
(b) Direct Nitriding Method of Aluminum(i.e., 2Al+N2=2AlN)
The direct nitriding method of aluminum has been used since Briegleb et al. succeeded first in the synthesis in 1862. Since the reaction is an exothermic reaction, the direct nitriding method has an advantage in that it is possible to produce aluminum nitride with ease by simply putting pure aluminum in a nitrogen stream.
However, in the direct nitriding method of aluminum, the supply of nitrogen into aluminum is shut off by a nitrided film when the surface of aluminum is covered with the nitrided film. When the supply of nitrogen is shut off, the nitriding reaction of aluminum is terminated. Accordingly, the direct nitriding method of aluminum has a disadvantage in that it is not possible to obtain aluminum nitride of 100% purity. Consequently, the nitriding is carried out industrially while aluminum is heated to a temperature of from 1,000 to 2,000° C.
Moreover, in the direct nitriding method of aluminum, nitriding and pulverizing the resulting aluminum nitride are carried out repeatedly in order to raise the reaction yield, or an additional treatment, such as adding AlF3 or AlN, is carried out in order to complete the reaction. However, since the bulks of aluminum nitride are hard, a variety of steps are required to pulverize them. As a result, the production cost goes up. Hence, the direct nitriding method of aluminum has a problem in that the resulting aluminum nitride is highly expensive.
U.S. Pat. No. 5,710,382 discloses a process for producing aluminum nitride by means of the direct nitriding method in which aluminum and aluminum nitride are used as raw materials. In the production process set forth in the U.S. Patent, two types of heating methods: namely, igniting the raw materials with an igniter; and heating them simply in a furnace, are disclosed as the method to initiate the nitriding reaction. According to the U.S. Patent, the peak temperature within the furnace and the peak temperature of a workpiece in the nitriding reaction are set at such a high temperature as from 1,845 to 2,115° C. when the raw materials are ignited with an igniter, and from 1,400 to 2,225° C. when they are heated with a furnace. Note that, in the nitriding reaction by heating with a furnace, the reaction initiation temperature is from 1,020 to 1,250° C. Accordingly, the peak temperature within the furnace and the peak temperature of the workpiece are much higher than the reaction initiation temperature. This is because the nitriding reaction is an exothermic reaction and the reaction heat of the nitriding reaction furthermore develops the nitriding reaction.
Thus, in the production process of aluminum nitride by means of the direct nitriding method, the peak temperature within the furnace and the peak temperature of the workpiece become high temperatures. Accordingly, the crystal growth and sintering of aluminum nitride occur. As a result, there is a problem in that the particle diameters of the resulting aluminum nitride particles are enlarged.